1. Field of The Invention
The present invention relates to nuclear fuel containing gadolinium wherein elemental gadolinium is introduced as a burnable poison. In particular, the present invention relates to nuclear fuel having increased fuel burnup efficiency, effected by increasing reactivity while minimizing any adverse effect on the shutdown margin and thermal margin.
2. Description of the Related Art
Natural gadolinium contains seven different isotopes. The isotopic composition and thermal neutron absorption cross-section of elemented gadolinium is shown in Table 1. In natural gadolinium, Gd-155 and Gd-157 have the largest cross-section and act as neutron absorbers, performing the function of reactivity control. Aside from the isotopes listed in Table 1, natural gadolinium also contains about 0.2% of Gd-152, but its effect on fuel reactivity is small.
TABLE 1 ______________________________________ Natural abundance Thermal neutron absorption Isotope (%) cross-section (barn) 10.sup.-24 cm.sup.2 ______________________________________ Gd-154 2 60 Gd-155 15 61000 Gd-156 20 2 Gd-157 16 255000 Gd-158 25 2 Gd-160 22 1 ______________________________________
FIG. 1 shows a transverse cross-sectional view of a conventional fuel assembly loaded in a boiling-water nuclear reactor. Fuel assembly (1) consists of a fuel rod bundle having 74 fuel rods (2) and two water rods (3) in the form of a lattice, this fuel rod bundle being surrounded by a channel box (4).
FIG. 2 is an axial cross-sectional view of a conventional fuel rod (2). The fuel rod (2) is rod-shaped having a diameter of about 1 cm, a total length of about 4 m, and is constituted by fuel pellets (5) consisting of sintered uranium(or plutonium) oxide packed in a cladding tube (6) made of zirconium alloy, fixed by a spring (7) and hermetically sealed by a top end plug (8) and a bottom end plug (9). The mean uranium enrichment of fuel assembly (1) in this example is 4.0%.
In fuel rods (10) indicated by the symbol G in FIG. 1, gadolinia (gadolinium oxide) is admixed with the uranium oxide as a burnable poison in order to control the initial excess reactivity of the reactor. In this example, gadolinia is contained in 14 of the 74 fuel rods.
FIG. 3 shows the infinite multiplication factor (11) of the fuel assembly of FIG. 1. The number of fuel rods (10) containing gadolinium is determined such that the initial excess reactivity of the reactor is within a suitable range and the gadolinia concentration is determined such that the gadolinia is consumed in the final period of the operating cycle, so as not to cause loss of reactivity. In this example, the operating period is assumed to be 13 months and the gadolinia concentration is 4.0%.
FIG. 4 shows the density of the number of atoms of the isotopes of gadolinium. As burnup proceeds, there is a rapid decrease in the amounts of Gd-155 and Gd-157, which have a large cross-section, and a slow decrease in Gd-154 and Gd-160, which have a small cross-section. Gd-156 and Gd-158 also have a small cross-section but since these are produced by neutron absorption of Gd-155 and Gd-157 respectively, they increase as burnup proceeds until the Gd-155 and Gd-157 have been converted, after which they change over to a slow decrease. Normally, the point at which the isotopes of large cross-section, namely Gd-155 and Gd-157, have decreased to practically constant values is referred to as gadolinia burnout.
FIG. 5 shows the neutron absorption factor of gadolinium isotopes. The change with burnup is roughly proportional to the atom density and its magnitude depends on the product of atom density and cross-section. Consequently, after gadolinia burnout, although Gd-156 and Gd-158 are present in higher atom density than the Gd-155 and Gd-157, since their cross-sections are smaller they provide neutron absorption of about the same order. The temporary increase in neutron absorption of Gd-155 in the initial period of burnup is caused by change in the neutron flux. Specifically, while a lot of Gd-157, which has a large cross-section, is still present, the neutron flux is decreased by the neutron absorption of this Gd-157. When the amount of Gd-157 is reduced as burnup proceeds, the neutron flux is thereby increased.
As already mentioned, the density of the gadolinia is determined such as not to produce a loss in reactivity when it is burned out at the latter part of the operating cycle. In this connection, the gadolinium isotopes that get burned up are Gd-155 and Gd-157, which have large cross-section; while the content of the other isotopes slightly decreases or increases. Consequently, even when all the Gd-155 and Gd-157 has been burned out, the gadolinium as a whole still retains neutron absorbance which produces a loss in reactivity.
In Table 2, the neutron absorption rate of each Gd isotope is given in 25 GWd/st, as a typical level of burnup after gadolinia burnout. The Tb-159 is produced by immediate beta decay of the Gd-159 generated by neutron absorption by Gd-158; its thermal neutron absorption cross-section is 23 barn. The total thermal neutron absorption factor due to these is 0.81%. This results in loss of reactivity, thus lowering the burnup efficiency of the fuel.
TABLE 2 ______________________________________ Isotope Neutron absorption factor (%) ______________________________________ Gd-154 0.04 Gd-155 0.05 Gd-156 0.21 Gd-157 0.23 Gd-158 0.21 Tb-159 0.05 Gd-160 0.02 Total 0.81 ______________________________________
If gadolinium is admixed with the uranium oxide as a burnable poison, the thermal conductivity is lowered, tending to produce higher fuel temperatures. Since the lowering in thermal conductivity is proportional to the gadolinium concentration, very high gadolinium concentrations are not desirable. There is also some concern that the fact that gadolinia-containing fuel rods have lower uranium concentrations than fuel rods not containing gadolinia may cause a drop in thermal output, avoiding excessive rise in fuel temperature.
A technique for increasing fuel burnup efficiency by reducing loss of reactivity is disclosed in Early Japanese Patent Publication Number Sho. 58-140673; wherein reactivity loss is decreased by raising the Gd-157 content. As an example of the application of this technique, Table 3 shows the reactivity loss at 25 GWd/st in the fuel assembly of FIG. 1 when gadolinium consisting solely of Gd-157 is used. Adjustment is made such that the total content of Gd-155 and Gd-157, which have large cross-sections, is the same as natural gadolinium, the gadolinia concentration being 1.2%. The fact that only a small degree of neutron absorption is produced by the Gd-158 generated by neutron absorption of the Gd-157 greatly reduces the overall loss of reactivity.
TABLE 3 ______________________________________ Isotope Neutron absorption factor (%) ______________________________________ Gd-154 0.0 Gd-155 0.0 Gd-156 0.0 Gd-157 0.02 Gd-158 0.16 Tb-159 0.03 Gd-160 0.0 Total 0.21 ______________________________________
The above technique made it possible to reduce the gadolinia concentration from 40%, when natural gadolinia is used, to 1.2%. Since thermal conductivity does not depend on the type of isotope, this means that the drop in thermal conductivity due to gadolinia inclusion is greatly mitigated. If this method is employed, the uranium enrichment of the fuel rods with gadolinia admixture can be raised above that used previously. This not only improves fuel economy by making it possible to raise the burnup rate but also reduces local power peaking in the fuel assembly cross-sectional plane. Alternatively, if it is assumed that a thermal conductivity of the same order as that obtained when natural gadolinium is employed is satisfactory, the concentration of the Gd-157, which is the isotope which essentially performs the burnable poison function, can be raised, thereby making it possible to lengthen the period of reactivity control by gadolinia by a factor of 3 or more. This makes possible long-term operation, so the availability factor of the reactor can be raised.
However, when applying the above technique to a fuel assembly, the following problems occur.
First, the reactor shutdown margin is adversely affected. On shutdown, the reactor is in a subcritical condition due to all the control rods being inserted into the core. However, the core is required to remain in subcritical condition, even if any one control rod is withdrawn from the core. This degree of subcriticality is the reactor shutdown margin.
FIG. 3 shows a comparison of the infinite multiplication factor (12) of a fuel assembly using gadolinium consisting of Gd-157 with the infinite multiplication factor (11) of a fuel assembly using natural gadolinium. When natural gadolinium, is employed, containing Gd-155 and Gd-157, which are of large cross-section, the infinite multiplication factor shows a comparatively gradual approach to its peak due to the different burnup rates of the two isotopes.
In contrast, in the case where only Gd-157 is present, a large sharp peak value is displayed. The shutdown margin becomes tight as the infinite multiplication factor approaches its peak, so if gadolinium of high Gd-157 content is employed, there is an adverse effect on the shutdown margin, i.e. the shutdown margin becomes too small.
The second problem is an increase in channel peaking. Channel peaking expresses the maximum value of the fuel assembly thermal output divided by the mean value of the fuel assembly thermal output. Fuel assemblies of different burnup are installed in the core of a reactor, and the fuel assembly thermal output depends on the infinite multiplication factor. Consequently, if the peak value of the infinite multiplication factor in fuel assemblies, using gadolinium of high Gd-157 content, gets too large, channel peaking also increases. As a result, the thermal margin of the minimum critical power ratio and/or the maximum linear heat generation rate, etc., is adversely affected.
The third problem is as follows: Since gadolinium has a large number of isotopes, the most effective method of isotope separation to increase the Gd-157 content is the laser method. In the laser method, natural gadolinium is irradiated with laser light having a specific wavelength to ionize only a specific isotope, which is recovered, thereby raising the content of that isotope. However, in the case of an isotope such as Gd-157, which is of odd mass number, some electron energy levels are split into several levels by interaction with the atomic nucleus. FIG. 6 shows an example of an optical absorption spectrum. When such an isotope is ionized, if a single laser beam is employed matching the wavelength of only one of these split energy levels, the ionization efficiency is decreased. In order to perform ionization efficiently, the number of laser beams may be increased corresponding to the number of split energy levels, but this complicates the laser device and raises its cost.